Question: Which of these numbers is prime? ${12,\ 27,\ 47,\ 63,\ 85}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 27 are 1, 3, 9, and 27. The factors of 47 are 1 and 47. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 85 are 1, 5, 17, and 85. Thus, 47 is a prime number.